1. Field of the Invention
The present invention relates generally to batteries, and more particularly to batteries that can include one or more of a block copolymeric electrolyte, lithium dichalcogenide compounds having a substantial amount of oxygen p-level characteristic at the Fermi energy level, and an electrode, one or more of which can be used with a lithium solid polymer electrolyte battery.
2. Background of the Invention
Rechargeable batteries enjoy an enormous and constantly growing global market due to their implementation in, for example, cellular phones, laptop computers and other consumer electronic products. In addition, the development of electrically-powered vehicles represents an immense potential market for these batteries. The increased interest in lithium intercalation compounds stems from their use in rechargeable batteries and, in particular, lithium solid state batteries.
Intercalation refers to a reaction in which ions, atoms or molecules penetrate between the layers of a solid material to form intercalation compounds. For example, alkali metal ions are known to insert between graphite layers to form intercalation compounds. Recently, dichalcogenides, such as dioxides and disulfides, have become increasingly popular for use in the intercalation of lithium ions. When dioxides are used, the overall reaction occurs as follows:(x2−x1) Li+(x2−x1) e−+Lix1MO2→Lix2MO2  (1) where M represents a metal or main group element and x2>x1≧0.In this reaction, lithium is placed in the structure of the dioxide without major changes to the structure.
The lithium solid polymer electrolyte rechargeable battery is an attractive technology for rechargeable battery applications due to its high predicted energy density, freedom in battery configuration, minimal potential for environmental and safety hazard, and low associated materials and processing costs. The lithium battery is charged by applying a voltage between the battery's electrodes, which causes lithium ions and electrons to be withdrawn from lithium hosts at the battery's cathode. Lithium ions flow from the cathode to the battery's anode through a polymer electrolyte to be reduced at the anode, the overall process requiring energy. Upon discharge, the reverse occurs; lithium ions and electrons are allowed to re-enter lithium hosts at the cathode while lithium is oxidized to lithium ions at the anode, an energetically favorable process that drives electrons through an external circuit, thereby supplying electrical power to a device to which the battery is connected.
The dioxide serves as lithium hosts in rechargeable batteries by intercalating lithium. The battery voltage derived from such an intercalation reaction depends on the difference in the chemical potential for lithium between the anode and cathode material:                               V          ⁡                      (            x            )                          =                                            -                                                μ                  Li                  cathode                                ⁡                                  (                  x                  )                                                      -                          μ              Li              anode                                            z            ⁢                                                   ⁢            F                                              (        2        )            where z is the electron transfer associated with Li intercalation, usually assumed to be equal to 1, and F is the Faraday constant. By integrating equation (2) between charged and discharged limit one obtains the average battery voltage arising from the intercalation reaction                               V          average                =                                            -              1                                                      x                2                            -                              x                1                                              [                                    E                                                Li                                      x                    ⁢                                                                                   ⁢                    2                                                  ⁢                M                ⁢                                                                   ⁢                                  O                  2                                                      -                          E                                                Li                  x1                                ⁢                                  MO                  1                                                      -                                          (                                                      x                    2                                    -                                      x                    1                                                  )                            ⁢                              E                Li                                                                        (        3        )            
The right-hand side of equation (3) is the energy associated with the formation of the discharged compound (Lix2MO2) from the charged compound (Lix1MO2). Hereafter x2 is set to 1, x1 is set to 0, and the right hand side of equation (3) is referred to as the “formation energy” of the intercalation compound LiMO2. The anode reference state is taken to be metallic Li although this has no significance for the results.
Currently known compounds such as LiCoO2 and LiMn2O4 have formation energies between 3 and 4 eV. For many applications a high voltage and low weight are desirable for the cathode as this leads to high specific energy. For example, for electrical vehicle applications the energy-to-weight ratio of the battery determines the ultimate driving distance between recharging.
With this goal in mind, the research into lithium intercalation compounds that has been conducted thus far has focused primarily on the synthesis and subsequent testing of various dioxide compounds. In preparing these compounds, workers have been guided by the conventional belief that, during intercalation of the lithium ion, the electron is transferred to the metal or main group atom of the dioxide. These efforts have led to the development of a variety of compounds, including LixCoO2, LixNiO2, LixMn2O4, and LixV3O13. In addition, LixTiS2 and other disulfides have been investigated for use in lithium intercalation. However, each of these compounds suffers from at least one shortcoming. For example, LixCoO2, LixV3O13 and LixTiS2 are relatively expensive to prepare. Moreover, LixNiO2 is comparatively difficult to process. Furthermore, LixMn2O4 possesses a limited capacity for providing energy.
Systems with multiple metals have been described in several patents and publications. Ohzuku, et al., “Synthesis and Characterization of Li Al1/4Ni3/4O2 for Lithium-Ion (Schuttle Cock) Batteries,” J. Electrochem. Soc., vol. 142, p. 4033 (1995), describe the mixed-metal composition of the title and report electrochemical properties thereof. The purpose of the preparation of the material, according to the authors, is to prevent overcharging-related degradation of a cathode.
Nazri, et al., in “Synthesis, Characterization, and Electrochemical Performances of Substituted Layered Transition Metal Oxides, LiM1-yM′yO2 (M′=Ni and Co, M′=B and Al),” Mat. Res. Soc. Symp. Proc., vol. 453, p. 635 (1997), describe addition of Al at various levels to LiNiO2 and LiCoO2 and investigation of related voltage change.
While the above and other reports represent, in some cases, interesting lithium compounds for electrochemical devices, on the whole the prior art is directed towards relatively high-temperature firing of compounds resulting in generally low-energy state products. For example, the above-cited reports do not appear to reflect the recognition or realization that LiAlO2 of the α-NaFeO2 structure has a higher formation energy than previously studied oxides such as LiCoO2 and LiNiO2, or that additions of LiAlO2 to another oxide of the α-NaFeO2 structure will raise the formation energy of said oxide. Instead, the results of Ohzuku et al. and Nazri et al. appear to show no significant voltage increase in batteries based on such compositions, which would discourage aspects of the present invention.
In general, many prior art mixed-metal compositions exhibit phase separation, and there is a general lack of appreciation that intercalation compounds of the present invention, described below, can play a role in high-energy electrochemical devices. Hence, it remains a challenge in the art to provide dichalcogenide compounds for use as lithium intercalation compounds that are relatively light-weight, inexpensive and easy to process and that have comparatively large formation energies. In addition, it is desirable to provide methods of predicting which dichalcogenide compounds may be most useful in lithium intercalation in order to reduce the time, effort and cost associated with the development of these compounds. Furthermore, methods must be provided for the synthesis and processing of these predicted compounds in the desired structure and with the desired homogeneity necessary to realize the predicted formation energies.
Development of commercially-viable lithium solid polymer electrolyte batteries has been hindered by complications, in particular complications involving the electrolyte. An inherent inverse relationship between ionic conductivity and dimensional stability exists in most known polymer electrolytes. That is, prior art electrolytes typically demonstrate good ionic conductivity, or good dimensional stability, but not both. Dimensional stability can be achieved by crosslinking, crystallization, glassification, or the like, but these arrangements generally impede ionic conductivity since conductivity requires a significant degree of polymer chain mobility.
For example, in linear chain polyethylene oxide (PEO) lithium salt electrolytes, crystallinity can severely hinder the mobility of the polymer chains, compromising room temperature ionic conductivities. Above the melting point of this system (Tm=65° C.), ionic conductivity increases significantly, but at these temperatures PEO behaves Theologically as a viscous fluid, losing its dimensional stability and hence its distinct advantage over liquid electrolytes that display much higher conductivities.
Since high ionic conductivity in PEO is characteristic of an amorphous state, most prior developmental efforts have concentrated on reducing crystallinity through the addition of plasticizers or modification of the polymer architecture through random copolymerization or the use of electrolytic pendant groups. However, these strategies generally have yielded materials with poor mechanical properties, i.e., materials that behave more like liquids than solids since, as crystallinity in PEO is reduced via these techniques, dimensional stability necessary for application in solid state batteries is compromised.
Crosslinking has been used as a technique for imparting mechanical rigidity to polymeric electrolytes, a common synthetic approach being to prepare network-type structures via irradiation or chemical crosslinking. The ionic conductivity of crosslinked systems is, however, inherently hindered by the presence of the crosslinks, as the crosslinks suppress chain mobility. Furthermore, crosslinked networks of solid polymer electrolyte materials do not flow and are insoluble, therefore multiple processing steps are required for preparation of electrolytes and arrangement of the electrolytes in batteries. Additionally, crosslinked materials tend to be non-recyclable.
Cathodes in state-of-the-art lithium solid polymer electrolyte batteries contain lithium ion host materials, electronically conductive particles to electronically connect the lithium ion hosts to a current collector (i.e., a battery terminal), and ionically-conductive particles to ionically connect the lithium ion hosts to a lithium-conducting polymer electrolyte. The lithium ion host particles typically are particles of lithium intercalation compounds. Typically, the electronically conductive particles are made of a substance such as carbon black or graphite, and the ionically conductive material is a polymer such as polyethylene oxide. The resulting cathode includes a mixture of particles of average size typically on the order of no less than about 100 microns.
For reliable operation, good contact between particles must be maintained to ensure an electronically-conductive pathway between lithium host particles and the external circuit, and a lithium-ion-conductive pathway between lithium host particles and the polymer electrolyte. In typical prior art arrangements, however, expansion and contraction of the mixture of particles occurring naturally during the course of charging and discharging, and due to temperature change of the environment in which the cathode is used, can result in loss of inter-particle contact, in particular, disconnection of the lithium host particle/electronically conductive particle interface. Moreover, repeated cycling often results in increased electrical resistance within the cathode due to passivation of the intercalation compound surface.
The available literature contains descriptions of a variety of solid polymer electrolytes. For example, Nagaoka, et al., in an article entitled, “A High Ionic Conductivity in Poly(dimethyl siloxane-co-ethylene oxide) Dissolving Lithium Perchlorate,” Journal of Polymer Science: Polymer Letters Edition, Vol. 22, 659-663 (1984), describe ionic conductivity in poly(dimethyl siloxane-co-ethylene oxide) doped with LiClO4. Bouridah, et al., in an article entitled, “a Poly(dimethylsiloxane)-Poly(ethylene-oxide) Based Polyurethane Networks Used as Electrolytes in Lithium Electrochemical Solid State Batteries,” Solid State Ionics, 15, 233-240 (1985) describe crosslinked polyether-grafted PDMS filled with 10 wt % LiClO4, and its ionic conductivity and thermal stability. Matsumoto, et al., in an article entitled, “Ionic Conductivity of Dual-Phase Polymer Electrolytes Comprised of NBR-SBR Latex Films Swollen with Lithium Salt Solutions,” J. Electrochem. Soc., 141, 8 (August, 1994) describe a technique involving swelling poly(acrylonitrile-co-butadiene) rubber and poly(styrene-co-butadiene) rubber mixed latex films with lithium salt solutions resulting in dual-phase polymer electrolytes.
The patent and academic literature contains descriptions of a variety of electrodes for polymer batteries. For example, Minett, et al. in “polymeric insertion electrodes, Solid State Ionics, 28-30, 1192-1196 (1988)” describe a mixed ionic/electronic conducting polymer matrix formed by exposing a film of polyethylene oxide soaked in pyrrole to aqueous FeCl3 solution or by exposing a film of FeCl3-impregnated polyethylene oxide to pyrrole vapor. Films were assembled into all-solid-state electrochemical cells using lithium as the anode and PEO8LiClO4 as electrolyte. U.S. Pat. No. 4,758,483 (Armand) describes a solid polymeric electrolyte that can be used in a composite electrode. The electrolyte is reported to include an ionic compound in solution in a copolymer of ethylene oxide and a second unit that is preferably an ethylene polyoxide structure including side-group radicals that introduce structural irregularities into the system reducing or eliminating crystallinity. A lithium salt, such as lithium perchlorate, is dissolved in the polymer system. Li and Khan, in an article entitled “Synthesis and properties of poly(2,5,8,11,14,17,20,23-octaoxapentacosyl methacrylate)-block-poly(4-vinylpyridine)”, Makromol. Chem. 192, 3043-3050 (1991) describe block copolymers of a soft, oxyethylene phase doped with LiClO4 and a hard, 4-vinylpyridine phase doped with a tetracyanoquinodimethane. The soft phase is rendered ionically conductive and the hard phase is rendered electronically conductive, and the copolymer can serve as a polymer electrode. The block copolymer shows microphase separation as indicated by the presence of two glass transition temperatures.
Significant effort has been directed toward viable solid polymer electrolytes, electrodes, and improved ion host particles, yet improvements are greatly needed. Therefore, it is an object of the present invention to provide lithium intercalation compounds that have reduced costs associated with their preparation and processing and that possess increased formation energies and lighter weight.
It is a further object of the present invention to provide methods of predicting which lithium intercalation compounds may be most useful in lithium batteries to decrease the effort and expense associated with the development of these compounds.
It is still another object of the invention to provide methods of processing lithium intercalation oxides with a high level of compositional homogeneity, as this is necessary to realize the increased formation energies of said compounds.
It is still another object of the invention to provide an electrolyte for batteries that exhibits good ionic conductivity, good dimensional stability, and that is easily processed.
It is still another object of the invention to provide an improved electrode for batteries that is dimensionally-stable, robust, that maintains good ionic conduction between the ion host and electrolyte and good electronic connection between the ion host and current collector after repeated cycling, and that is easily and economically manufactured.